A geodetic baseline is a measurement standard for length in geodesy. Typically, it is a few hundred metres long measured distance in terrain having observation pillars on underground benchmarks. Its length is metrologically traceable to the definition of the metre, the SI-unit of length. The most famous and most accurate of the baselines is the 864 metres long Nummela Standard Baseline maintained by the Finnish Geodetic Institute (FGI). Since year 1947, the length of the baseline has varied only ±0.6 mm in altogether 16 measurements with an uncertainty of the order of 0.1 mm for a single measurement. Earlier the Nummela baseline gave the scale of mapping in Finland, but nowadays it is used for calibration of the most accurate electronic distance measurement instruments. Using such instruments, the scale of the Nummela Standard Baseline has been transferred to ten different countries around the world during the last ten years. This summer a measurement campaign was held in Germany, in which the scale was transferred to the baseline of PTB (PTB, Physikalisch-Technische Bundesanstalt, national metrology institute in Germany).
The accuracy of measurements in Nummela is based on the Väisälä interference comparator, an instrument using white light interferometry and invented by academician Yrjö Väisälä already in the 1930s. Despite of the huge technological advancement in the last years, the comparator still beats in accuracy state-of-the-art electronic distance measurement instruments in long distance measurements. Measurement of baselines with Väisälä comparator take advantage of quartz gauges, i.e. one metre long bars made of quartz and having spherical end surfaces. The quartz gauges give an accurate scale for the Väisälä comparator.
The quartz gauges of FGI are calibrated at MIKES and are traceable to the SI-metre. These gauges in turn bring the traceable scale to Väisälä interference comparator and to the Nummela Standard Baseline. The quartz gauge number VIII is used in measurements of the Nummela baseline and its length is known from absolute calibrations and intercomparisons with a standard uncertainty of 35 nm. In a Väisälä interference comparator, the length of the gauge is multiplied to correspond to the distance between mirrors placed on observation pillars in the baseline. This distance is then transferred by projection measurements to the baseline lengths between underground benchmarks.
Four quartz gauges of FGI have recently been calibrated absolutely at MIKES. The calibration was performed by using MIKES’ long gauge block interferometer that utilizes white light and 633-nm laser light interference patterns. By using white light, beforehand knowledge of the length of the end standard is not required. The traceability to gauge block calibrations is achieved by calibrating the wavelengths of lasers used in the interferometers against national measurement standards of length, iodine-stabilised He-Ne lasers.
Further information: Assistant Research Scientist Pasi Laukkanen, firstname.lastname@example.org, tel. +358 29 505 4472
Prof. Markku Poutanen, email@example.com, tel. +358 29 530 8070